Most of the fertilizers, soils and additives you use in the garden are labeled with an “NPK” value.  But what is this mysterious combination of numbers and how does it affect your garden?

3 Main Elements

N-P-K are elemental symbols which stand for: nitrogen, phosphorus, and potassium.  These 3 elements are considered the major macronutrients your soil needs in order to thrive.  So the values given to each element corresponds to how much of that element are in the garden product.

What is NPK?

These values do not directly tell you the nutrient contents of the garden product.  Rather, the NPK values represent the percentage of the elemental nutrient in its most dominant form by the garden product’s total weight.  This may be a little confusing; so we broke it down for you.

How Do I Use NPK Values?

You can convert the NPK values to show the actual content of nitrogen, phosphorus, and potassium you are about to add to your soil. Nitrogen values represent actual nitrogen content and do not need to be converted.  So an 18-0-0 would have 18% elemental nitrogen.

Potassium most dominant form is P₂O₅.  So the second number will be the percentage of P₂O₅ by weight.  You can find the value of elemental phosphorus by compensating for the amount of oxygen in P₂O₅.  P₂O₅ is 43.6% nitrogen and 56.4%.  Multiply the value on the bag (percentage of P₂O₅) by 0.436 to get the amount of elemental phosphorus you are adding to your soil.

The same concept applies to the K or potassium value.  Potassium is stored as K₂O.  K₂O is 17% oxygen and 83% potassium.  Multiply the K value shown on the bag (actually the K₂O percentage) by .83 to get the amount of elemental potassium you are adding to your soil.

NPK Conversion Ratios

Here is a condensed form of the conversion ratios:

• N is nitrogen. No conversion.
• P is P₂O₅.  Multiply by 0.436.
• K is K₂O.  Multiply by 0.83.

Remember that over time, nutrients will be used up, leached out and washed away.  So check the nutrient contents of your soil often and adjust accordingly.  There are ideal levels for each of these macronutrients in your gardens.  Adding too much nutrients to the soil will cause water pollution and “burn” your plants.  So know your soil, know your plants, and know your garden products for a bountiful harvest this year.

Now that you know what these values are and how to convert these numbers into macronutrient ratios, stay tuned for how these macronutrients affect plant health.

 

Photos: SuSanA Secretariat,

By Jake Frazier

Jake Frazier is an outdoor enthusiast and the owner of Residential Ecology, a sustainable ecological resource management company. He uses existing natural systems to improve the quality of life for both humans and the Earth. Jake is interested in permaculture, living systems and exploring. Connect with him on LinkedIn.